1. Field of Invention
This invention relates to a displacement transducer and, more particularly, to a displacement transducer which utilizes light to detect mechanical displacement.
2. Description of Prior Art
FIG. 1 depicts a conventional displacement transducer comprising a code disk 1, a code piece 2, light emitting element 3, and light receiving element 4. A plurality of light permeable units are evenly spaced around the circumference of code disk 1. The four light permeable unit groups 2.sub.1 through 2.sub.4 are formed on code piece 2 so that they are phase shifted respectively by 90.degree. with respect to the light permeable unit of the code disk 1. Each of the light permeable groups 2.sub.1 through 2.sub.4 of code piece 2 consists of a plurality of light permeable units (three light permeable units in FIG. 1) formed with the same space as the light permeable unit of code disk 1. Light emitting element 3 comprises four elements disposed in order that each of the light permeable unit groups 2.sub.1 through 2.sub.4 is illuminated separately. Code disk 1 and code piece 2 are arranged so that they are relatively rotatable. Code piece 2, light emitting element 3 and light receiving element 4 are fixedly arranged so that code piece 2 and light emitting element 3 are opposite to light receiving element 4 through each of the light permeable unit groups 2.sub.1 through 2.sub.4 of code piece 2.
In order to detect an amount of rotation, four light emitting elements 3.sub.1 through 3.sub.4 are simultaneously turned ON and the output signals from the light receiving elements 4.sub.1 through 4.sub.4 are digitally parallel processed by means of logic circuits.
It is difficult, however, according to the conventional arrangement above described, to obtain a resolution power exceeding the space of the light permeable units formed on code disk 1 and code piece 2. Also, it is difficult to obtain a response faster than the response speed of light receiving element 4.
In a presently marketed encoder using phototransistors as the light receiving elements 4, for example, the upper limit of the response frequency is approximately 50 kHz. That is, when the encoder is combined with a DC motor which causes code disk 1 and code piece 2 (for producing 1000 pulses per revolution) to be rotated at a speed of 6000 rpm, the frequency will be 100 kHz which is insufficient for satisfactory detection. In addition, code disk 1 and code piece 2 (for producing 500 pulses or more per revolution) in a small sized encoder require highly accurate machining techniques and result in high cost.
Also, the conventional device causes problems such as uneven light intensity, uneven light receiving sensitivity, etc, since a plurality of light emitting elements and a plurality of light receiving elements are used.